This application claims priority to provisional application U.S. Ser. No. 60/507,184, titled “Flow Monitoring Using Flow Control Valve”, filed Sep. 29, 2003.
The flow rate of a fluid into a process is important for many applications, because it provides useful information to both the user of the fluid in an application and the supplier of the fluid. For cryogenic fluid applications knowing the flow rate into an application will allow the user to know the amount and costs in real time for the use of the fluid. Today, the user waits for an invoice after refilling a cryogenic storage tank to determine past usage and to allocate costs after the fact. The user would also benefit from knowing the flow rate which would give extra information on performance of the equipment, particularly during commissioning (start-up) of the equipment. The flow rate of the cryogenic fluid indicates the refrigeration power and allows the performance of the application to be optimized to provide best efficiency. With flow rate data it will be possible to immediately demonstrate improvements made to equipment and measure savings.
The fluid flow meters that are currently available are not generally used for cryogenic fluid applications. Flow meters useful for cryogenic fluids are expensive and sophisticated pieces of equipment, and their accuracy depends on the consistency of the gas and liquid fractions of the fluid. Many flow meters are accurate for subcooled and low gas fraction two phase flows (less than 0.5% gas by weight), but are poor for higher gas fraction two phase fluids, and gas only fluids; however, in cryogenic applications, the gas and liquid fractions in the fluid may vary greatly. Due to the flow meters' high sensitivity, they are also susceptible to rough handling. Also the potential for moisture ingress is a major cause for unreliability with conventional low temperature flow metering equipment.
The nature of cryogenic flow gives rise to three distinct flow regimes: sub cooled liquid, two phase gas and liquid, and super heated gas. Flow meters are normally designed to work in one of these regions. To maintain good accuracy the flow meters are complex, sensitive and expensive, but their accuracy can be poor and highly variable when the gas and liquid fractions of the cryogenic fluid vary beyond the gas and liquid fractions that the meters are designed for.
Current flow meters include those that measure the flow directly using the velocity and density of the fluid, and those that measure the flow indirectly using properties such as pressure drop and fluid modeling equations. The flow meters that measure the flow directly are complex meters. They are accurate for their design conditions. Examples of these meters are disclosed in U.S. Pat. Nos. 4,835,456, 3,635,084, 4,272,982. Flow is measured using a moving part (e.g. a turbine) to find the fluid velocity directly. They are susceptible to the stability of the gas and liquid fractions of the cryogenic fluid and can have poor accuracy for high gas fraction cryogenic fluids.
Non-cryogenic indirect measurement flow meters include Pitot tubes, and the Venturi's and orifice plate meters. They rely on finding the pressure drop and using it in flow modeling equations, for example Bernoulli's equation. For two phase flow more complex flow modeling is required and is very dependent on the gas and liquid fractions of the cryogenic fluid. An example can be found in U.S. Pat. No. 4,168,624.
Other examples of flow meters measure flow using electronic means (e.g. capacitance) to find the fluid velocity, such as those flow meters disclosed in U.S. Pat. Nos. 4,509,366, and 5,861,755. They are susceptible to the stability of the gas and liquid fractions of the cryogenic fluid and can have poor accuracy for high gas fraction cryogenic fluids.
A third group of flow meters arises where the fluid is split into its gas and liquid phase, each phase has its flow rate measured using one of the techniques above and the total flow is found by summing the two. Examples of these flow meters are found in U.S. Pat. Nos. 4,881,412 and 5,679,905. They are complex and expensive as they require two or more flow meters to measure the total flow.
There is a need in the art to provide a better way to measure the flow rate of a cryogenic fluid.